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Code Issues 151 Wiki Activity

multiplayer: whoever drives a train runs it, and their ride is the smooth one

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Until now the server ran the physics of everything and a client re-ran it from
the server's frame times, corrected a few times a second. That is the worst of
both: the client's own train never moved smoothly, its controls answered a
round trip late, and the two copies still sat metres apart between corrections.

Simulation authority now follows the train:

- the first person aboard a train runs its physics on their own machine, and
  what they produce is what the rest of the session is told about it. A train
  with nobody in it belongs to the host. Every peer works the owner out the
  same way, from the crew of its lowest numbered car, so nobody has to be told
- a peer never applies corrections to a train it runs itself. Its own physics
  is the authority on it, and overwriting that is precisely what made the
  owner's ride stutter
- a client publishes the state of its own trains upstream, ten times a second.
  The server takes it, filtered so a peer can only move the trains it is
  entitled to move, and passes it on to everybody else in its own stream
- so the picture is: your train is smooth for you and may stutter for the
  others; their trains may stutter for you and are smooth for them

Two things follow from it:

- commands for the train you run take effect the moment you press the key
  instead of after a trip to the server and back. They are still sent, so
  everybody else sees them, and the echo is dropped when it returns
- the client no longer replays the server's frame times out of a jitter buffer.
  It runs on its own clock, like the host does, and simply carries out the
  commands as they arrive. Playing back somebody else's frame times meant the
  world ran at the wrong speed whenever the two machines drew at different
  rates, and every buffered frame sat between the player and their controls

Also: the server only sends a frame when it has commands in it, or every tenth
one to carry the tick, instead of one per rendered frame.
This commit is contained in:
Hirek
2026-09-07 00:19:40 +00:00
parent 0b75e701fc
commit c0dbf99f1a
10 changed files with 230 additions and 130 deletions
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102
network/snapshot.cpp
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@@ -33,7 +33,7 @@ uint32_t const SNAPSHOT_MAGIC{0x4e535545}; // 'EUSN'
// a joining peer is placed exactly; a running one is left to its own physics for small
// errors, because nudging a vehicle every fraction of a second is worse than the error
double const REPOSITION_TOLERANCE_JOIN{0.5};
double const REPOSITION_TOLERANCE_CORRECTION{2.5};
double const REPOSITION_TOLERANCE_CORRECTION{1.5};
// ---------------------------------------------------------------------------
// what the authority last sent, so that a routine correction only carries changes
@@ -355,41 +355,20 @@ vehicle_state read_from(TDynamicObject const &Vehicle)
return state;
}
// true for the train the local player is working. its controls are driven by the commands
// its crew issue, which every peer already receives, so the correction must keep its hands
// off them - otherwise it keeps snapping a lever back a fraction of a second after the
// player moved it, and the control looks broken
bool is_own_train(std::string const &Name)
{
auto const seat = network::Crews.vehicle_of(Global.network_peer_id);
if (seat == network::ENTITY_NONE)
return false;
auto const train = network::Entities.consist_of(network::Entities.id_of(Name));
return (train != network::ENTITY_NONE) && (network::Entities.consist_of(seat) == train);
}
// applies everything except where the vehicle is; position is dealt with per consist,
// because moving one vehicle of a coupled set on its own tears the couplers apart
// because moving one vehicle of a coupled set on its own tears the couplers apart.
// only ever called for vehicles somebody else is running
void apply_controls(TDynamicObject &Vehicle, vehicle_state const &State)
{
auto &mover = *Vehicle.MoverParameters;
// what the physics produced is always taken from the authority; what a person set is
// not, on the train that person is working
bool const ours = is_own_train(State.name);
mover.V = State.velocity;
mover.Vel = State.velocitykmh;
if (!ours)
{
mover.DirActive = State.diractive;
mover.MainCtrlPos = State.mainctrl;
mover.ScndCtrlPos = State.scndctrl;
mover.BrakeCtrlPos = State.brakectrl;
mover.LocalBrakePosA = State.localbrake;
}
mover.DirActive = State.diractive;
mover.MainCtrlPos = State.mainctrl;
mover.ScndCtrlPos = State.scndctrl;
mover.BrakeCtrlPos = State.brakectrl;
mover.LocalBrakePosA = State.localbrake;
mover.PipePress = State.pipepress;
mover.BrakePress = State.brakepress;
@@ -398,14 +377,6 @@ void apply_controls(TDynamicObject &Vehicle, vehicle_state const &State)
mover.Compressor = State.compressor;
mover.CompressedVolume = State.compressedvolume;
if (ours)
{
// the rest of it is what the crew set, and their commands reach us on their own
if (Vehicle.Mechanik != nullptr && Vehicle.Mechanik->AIControllFlag != State.aiactive)
Vehicle.Mechanik->TakeControl(State.aiactive);
return;
}
// the appliances go through the vehicle's own switches wherever it has them, so that
// whatever they drag along stays consistent. range local: every vehicle carries its
// own state in the update, there is no need to push it down the consist twice
@@ -470,7 +441,7 @@ void reposition(TDynamicObject &Vehicle, vehicle_state const &State)
Vehicle.place_on_track(track, nose, false);
}
std::string write_vehicles(bool const Full)
std::string write_vehicles(bool const Full, bool const OwnedOnly)
{
std::ostringstream entries;
uint32_t count{0};
@@ -480,6 +451,9 @@ std::string write_vehicles(bool const Full)
if (vehicle == nullptr || vehicle->MoverParameters == nullptr)
continue;
if (OwnedOnly && !network::is_locally_simulated(network::Entities.id_of(vehicle->name())))
continue;
auto const state = read_from(*vehicle);
std::ostringstream packed;
@@ -516,7 +490,7 @@ std::string write_vehicles(bool const Full)
return body.str();
}
void read_vehicles(std::istream &Stream, network::snapshot_mode const Mode, network::snapshot_result &Result)
void read_vehicles(std::istream &Stream, network::snapshot_mode const Mode, network::PeerId const Owner, network::snapshot_result &Result)
{
auto const count = sn_utils::ld_uint32(Stream);
double const tolerance = (Mode == network::snapshot_mode::join ? REPOSITION_TOLERANCE_JOIN : REPOSITION_TOLERANCE_CORRECTION);
@@ -532,6 +506,18 @@ void read_vehicles(std::istream &Stream, network::snapshot_mode const Mode, netw
if (vehicle == nullptr || vehicle->MoverParameters == nullptr)
continue;
auto const entity = network::Entities.id_of(state.name);
// a train this peer runs is not corrected by anybody: its own physics is what the
// rest of the session is being told about, and overwriting it would be the thing
// that makes the owner's ride stutter
if ((Mode == network::snapshot_mode::correction) && network::is_locally_simulated(entity))
continue;
// and a peer may only move what it is entitled to move
if ((Owner != network::PEER_NONE) && (network::simulation_owner(entity) != Owner))
continue;
apply_controls(*vehicle, state);
states.emplace(vehicle, state);
@@ -793,10 +779,28 @@ void network::reset_snapshot_history()
g_lastswitches.clear();
}
std::string network::take_snapshot(bool const Full)
std::string network::take_snapshot(bool const Full, bool const OwnedOnly)
{
auto const vehicles = write_vehicles(Full, OwnedOnly);
if (OwnedOnly)
{
// what a peer says about its own trains, and nothing else: the clock, the crews,
// the memory cells and the switches all belong to the authority
if (vehicles.empty())
return std::string();
std::ostringstream owned;
sn_utils::ls_uint32(owned, SNAPSHOT_MAGIC);
sn_utils::ls_uint32(owned, SNAPSHOT_VERSION);
sn_utils::ls_uint64(owned, Global.simulation_tick);
sn_utils::ls_uint32(owned, 1);
write_chunk(owned, SNAPSHOT_VEHICLES, vehicles);
return owned.str();
}
auto const session = write_session(Full);
auto const vehicles = write_vehicles(Full);
auto const crews = write_crews();
auto const memcells = write_memcells(Full);
auto const switches = write_switches(Full);
@@ -839,7 +843,7 @@ std::string network::take_snapshot(bool const Full)
return blob;
}
network::snapshot_result network::apply_snapshot(std::string const &Blob, snapshot_mode const Mode)
network::snapshot_result network::apply_snapshot(std::string const &Blob, snapshot_mode const Mode, PeerId const Owner)
{
snapshot_result result;
@@ -880,19 +884,23 @@ network::snapshot_result network::apply_snapshot(std::string const &Blob, snapsh
switch (id)
{
case SNAPSHOT_SESSION:
read_session(chunk, Mode);
if (Owner == PEER_NONE)
read_session(chunk, Mode);
break;
case SNAPSHOT_VEHICLES:
read_vehicles(chunk, Mode, result);
read_vehicles(chunk, Mode, Owner, result);
break;
case SNAPSHOT_CREWS:
read_crews(chunk);
if (Owner == PEER_NONE)
read_crews(chunk);
break;
case SNAPSHOT_MEMCELLS:
read_memcells(chunk);
if (Owner == PEER_NONE)
read_memcells(chunk);
break;
case SNAPSHOT_SWITCHES:
read_switches(chunk);
if (Owner == PEER_NONE)
read_switches(chunk);
break;
default:
// a section this build knows nothing about; its length is right there, so it